This invention relates generally to treatment of selected tissue by inter-vivo radiation, specifically to radiation treatment of selected regions of the cardiovascular system that have been subjected to trauma to prevent restenosis of the traumatized region, more specifically to radiation treatment to prevent restenosis of an artery traumatized by percutaneous transluminal angioplasty (PTA).
PTA treatment of the coronary arteries, percutaneous transluminal coronary angioplasty (PTCA), also known as balloon angioplasty, is the predominant treatment for coronary vessel stenosis. Approximately 300,000 procedures were performed in the United States (U.S.) in 1990 and an estimated 400,000 in 1992. The U.S. market constitutes roughly half of the total market for this procedure. The increasing popularity of the PTCA procedure is attributable to its relatively high success rate, and its minimal invasiveness compared with coronary by-pass surgery. Patients treated by PTCA, however, suffer from a high incidence of restenosis, with about 35% of all patients requiring repeat PTCA procedures or by-pass surgery, with attendant high cost and added patient risk. More recent attempts to prevent restenosis by use of drugs, mechanical devices, and other experimental procedures have had limited success.
Restenosis occurs as a result of injury to the arterial wall during the lumen opening angioplasty procedure. In some patients, the injury initiates a repair response that is characterized by hyperplastic growth of the vascular smooth muscle cells in the region traumatized by the angioplasty. The hyperplasia of smooth muscle cells narrows the lumen that was opened by the angioplasty, thereby necessitating a repeat PTCA or other procedure to alleviate the restenosis.
Preliminary studies indicate that intravascular radiotherapy (IRT) has promise in the prevention or long-term control of restenosis following angioplasty. It is also speculated that IRT may be used to prevent stenosis following cardiovascular graft procedures or other trauma to the vessel wall. A proposed IRT method is first to advance a flexible catheter (radioguide catheter) through the cardiovascular system of the patient until the distal tip is at or near the region of the vessel that has been subjected to the angioplasty procedure. Subsequently, a treatment catheter, comprising a wire or small catheter having a radiation source at the tip (hereinafter referred to as a source wire), is advanced through the radioguide catheter until the radiation source is disposed at the affected region. The radiation source is held at the affected region for a predetermined treatment period calculated to deliver an effective dose of radiation, then is withdrawn.
A principal shortcoming in current IRT methods and apparatus, however, is the lack of any provision to control the radial location of the radioactive source within the lumen during treatment. The effective dose to inhibit smooth muscle cell hyperplasia and the resulting restenosis is approximately 1,000-3,000 rads. For a given source activity, the intensity of the radiation drops rapidly as a function of the distance from the source. Accordingly, if the source is not held reasonably near the center of the lumen, for a given treatment period, the portion of the vessel wall nearest the source will receive an excess dose of radiation, while the portion of the vessel wall farthest from the source will receive less than the prescribed dose. Overdosing of a section of blood vessel can cause arterial necrosis, inflammation and hemorrhaging. Underdosing will result in no inhibition of smooth muscle cell hyperplasia, or even exacerbation of the hyperplasia and resulting restenosis.
Current IRT methods do not provide for centering of the radiation source within the lumen to provide substantially uniform irradiation. U.S. Pat. No. 5,199,939 to Dake et al. teaches the use of a catheter having a radiation source assembled at the tip for treatment of vascular lumens to prevent restenosis. This patent does not, however, teach centering to provide uniform irradiation of the walls of the vessel during treatment.
U.S. Pat. No. 5,302,168 to Hess also discloses use of a radioactive source positioned within the stenosed region of an artery to inhibit restenosis. This patent, however, does not address non-uniform irradiation as a potential problem, nor does it teach centering to provide uniform irradiation.
U.S. Pat. No. 5,213,561 to Weinstein et al. also does not address non-uniform irradiation as a potential problem and does not teach centering of the radioactive source as a method of providing uniform irradiation. In one embodiment, the radioactive material is placed on a catheter tube inside the balloon of a balloon catheter, with a retractable radiation shield surrounding the source so that the shield may be retracted to expose the source immediately following completion of angioplasty. Although the balloon catheter of this embodiment has an internal tube, the tube is not constrained to be positioned at the center of the balloon. Moreover, the patent does not contain a teaching of centering the radioactive material. In fact, the relevant disclosed embodiment shows the radioactive crystals mounted eccentrically on the tube.
H. Bottcher, et al. of the Johann Wolfgang Goerhe University Medical Center, Frankfurt, Germany reported in November 1992 of having treated human superficial femoral arteries with a calculated dose of 12 Gray (1,200 rads) from a 10 Curie source. Bottcher, et al. recognized that the theoretical dosage from a fixed radiation source within the vessel varied with distance from the source. However, Bottcher, et al. do not teach centering as a means to prevent inconsistent irradiation of the vessel walls as a result of distance variations. Instead, Bottcher, et al. teach that the dynamic process of the catheter floating in the relatively straight femoral artery lumen mitigates the tissue damage anticipated from the inconsistent irradiation.
Accordingly, it is a principal object of the present invention to provide a method and apparatus for intravascular radiotherapy in which a uniform dose of radiation is delivered to the wall of the blood vessel by centering the radioactive source within the lumen. A further object of the present invention is to provide for a compliant centering means that will conform to the tortuous curves in and around the coronary arteries while maintaining the radioactive source at the center of the artery for the entire length of the region being treated.
Because current IRT methods and apparatus do not provide for centering the radiation source within the lumen, they also do not address centering the radiation source without obstructing the flow of blood through the blood vessel being treated. Obstruction of a coronary artery for a prolonged period of type, typically for more than approximately one minute may cause impairment of heart function or irritability of the heart and may result in suffer severe ischemia, angina, cardiac arrest, myocardial infarction (a heart attack), and/or shock. The radiation sources used for IRT are preferably of relatively small mass and size. Typically, to deliver an effective dose, these sources must remain centered in the target area for a minimum of approximately 4 minutes. Any means for centering the radiation source that blocked the flow of blood could only be used in a coronary artery for less than about one minute at a time, necessitating either multiple repeat irradiation treatments with intervening periods of perfusion, or use of a less preferable larger source.
Accordingly, another related object is to provide a radioactive source centering method and apparatus that allows per fusion of blood past the radiation source during the treatment period.